This invention relates to a method of welding ferritic and austenitic stainless steels. More particularly, it relates to a method of welding such steels with a stabilized filler metal to provide a corrosion-resistant article.
Although conventional ferritic stainless steels could provide adequate corrosion resistance in some environments, the steels are not extensively used in an as-welded condition because of difficulties in forming and joining. In joining conventional ferritic stainless steels, the as-welded condition often exhibits a tendency to form hard brittle martensite and to show poor corrosion resistance. Such steels are subject to sensitization as a result of the reaction of carbon, nitrogen and chromium in the formation of carbides and nitrides. There are two conventionally-known methods to avoid or reduce sensitization in the as-welded condition. The alloy may be stabilized, such as by the addition of titanium, columbium, zirconium or mixtures of these alone or in combination with aluminum in carefully controlled amounts. Sensitization can also be reduced by lowering the carbon and nitrogen levels of the alloy such as E-Brite alloys (trademark of Allegheny Ludlum Steel Corporation) and alloys containing nominally 29% chromium and 4% molybdenum which may or may not include 2% nickel. However, some alloys containing low carbon and nitrogen such as the E-Brite alloys (nominally 26% chromium) may still need the addition of columbium as a stabilizer, for example, since the solubility of carbon and nitrogen in the ferritic steel is very low. Higher alloy steels like the 29% chromium and 4% molybdenum alloys seem to tolerate generally higher levels of carbon and nitrogen and do not require stabilizers.
As a result of such techniques, newer ferritic alloys have been developed which have a corrosion resistance comparable to austenitic stainless steels and some nickel base alloys such as Alloys G, 825 or 625. Such new ferritic stainless steels may have low interstitials (relatively low carbon and nitrogen levels) or be stabilized, or both.
Though the ferritic stainless steels are readily weldable and have good mechanical and corrosion properties, it is frequently necessary to join dissimilar alloys, such as by welding. As a result, in some corrosive media particular intergranular corrosion attack occurs, which is not present if welding occurs between low interstitial alloys which are matched or stabilized ferritic alloys which are matched. Examples of welds between a ferritic stainless steel and an austenitic stainless steel which are in service in a corrosive environment occur particularly in applications of a ferritic steel tube disposed in an austenitic steel tubesheet. Such an arrangement may occur because of the corrosion service required of the tubes and the ready availability of the austenitic steel tubesheet. Such arrangments are also occasioned when retrofitting existing austenitic steel tubesheets with ferritic steel tubes to satisfy corrosion service requirements. The seal weld to join the ferritic stainless steel tube to an austenitic stainless steel tubesheet may or may not require the use of a similar material in the form of a filler material. In any event, however, the corrosion resistance of the combinations vary depending upon the two alloys, any filler metal and the corrosive environment. The use of a ferritic steel with low solubility for carbon and nitrogen in comparison to austenitic stainless steels makes the production of a weld joint resistant to intergranular corrosion more complex than an all austenitic stainless steel combination. In a ferritic-austenitic combination or composite article, the ferritic steel is the primary cause for any reduction or detrimental effect to the intergranular corrosion resistance of the composite weld.
It is known that a stabilized filler metal, such as Inconel 82 (trademark of Huntington Alloys, Inc.) alloy, nominally containing approximately 20% chromium, 80% nickel and 3% titanium and columbium combined, may be used to sequester the carbon. An article entitled "Welding E-Brite 26-1 to Other Alloys" by R. Lowrie, Welding Journal, November 1973, discloses using that stabilized filler for welding E-Brite 26-1 alloy to Type 316 austenitic alloy. There it is explained that the weld of E-Brite alloy to other materials, being a combination between the filler metal and the two base metals, one of which is generally austenitic, is unlikely to be as corrosion resistant as the fully ferritic or nearly fully austenitic base metals because of the very low solubilities for carbon or nitrogen in the ferritic stainless steels, the generally higher carbon and nitrogen levels in the austenitic steels and the precipitation of chromium carbides at grain boundaries below 1200.degree. F., such as may occur during cooling from weld heat. This was cited to result in chromium depletion and some loss of corrosion resistance. The article further reports that the welds of E-Brite 26-1 alloy to Type 316 made using Inconel 82 filler pass the Strauss intergranular corrosion test and a general corrosion resistance test in acetic acid. The best resistance in a weld for stress corrosion, however, was obtained with a nonstabilized filler metal of Type 308L alloy, presumably because of a large volume fraction of ferrite in the weld. This combination is not mentioned as passing the Strauss test.
Current study suggests that a weld between E-Brite 26-1 alloy and Type 316 or 316L made using 308L filler would be susceptible to attack in standard intergranular corrosion resistance tests. The Lowrie article notes that the combinations which passed the Strauss test had not produced a carbide network in either the weld or E-Brite 26-1 alloy heat-affected zone. However, Table 6 of the paper (microstructures of welds) shows other combinations including the 308L filler which passed the stress corrosion test but which also show precipitated carbides which presumably the Strauss test would measure with a fail rating. The article generally recommended that dissimilar metal joints should not be designed for corrosive environments which could cause intergranular attack or chloride stress corrosion cracking.
It is also known that when columbium is the principal stabilizer, some titanium is used to avoid weld cracking sensitivity. Titanium, however, may be used as the sole stabilizer.
What is needed is a method for optimizing the corrosion resistance of a weld between a ferritic stainless steel and an austenitic stainless steel having a total carbon and nitrogen content which exceeds that of the ferritic stainless steel. Furthermore, it is desirable to optimize the method for welding low interstitial ferritic stainless steels to an austenitic stainless steel. The welds of such alloys should satisfactorily pass applicable intergranular corrosion tests such as those described in ASTM Procedures A262, A708 or A763. It is also an object of the present invention to provide a welded article having seal welds or strength welds suitable for a ferritic stainless steel tube-austenitic stainless steel tubesheet welded composite article.